A speech detection device is known from the document U.S. Pat. No. 5,321,745 and takes the form of a telephone system. The known telephone system is connected to the public telephone network and is adapted to receive an input signal containing tone signals and/or speech signals.
The telephone system includes a tone detection device adapted to detect a tone signal in an input signal containing signaling tones. A signaling tone can be formed, for example, by a CNG tone (calling tone) having a frequency of 1.1 kHz and characterizing an incoming fax signal in the input signal. When the tone detection device detects a CNG tone the input signal is transferred to fax means of the telephone system.
When a received signal contains a speech signal which at given instants also contains frequency components having a frequency of 1.1 kHz, the speech detection means can erroneously detect a CNG tone, upon which a speech signal is transferred to the fax means. In order to preclude an erroneous detection of a signaling tone in a received signal the known telephone system includes a speech detection device.
The speech detection device has processing means which perform a fast Fourier transform upon an input signal and which check the frequency domain of the input signal. For this purpose, the processing means determine the three largest amplitudes M1, M2 and M3 of the transformed input signal and the frequencies f1(M1), f2(M2) and f3(M3) at which these amplitude values occur.
Threshold value test means of the speech detection device check whether the frequencies f1(M1) and f2(M2) are higher than the frequency threshold value of 300 Hz and whether M1/M3 is greater than a coefficient threshold value. The threshold value test means supply detection information corresponding to the results of the tests.
Evaluation means of the speech detection means evaluate the detection information and, when necessary, they supply speech detection information which characterizes the presence of a speech signal in the input signal. When speech detection information is present the input signal is not transferred to the fax means of the telephone system, even when the tone detection means have detected a CNG tone.
The known speech detection device has the drawback that a very high computing power is required in order to carry out a fast Fourier transform, for which reason the known speech detection device requires the use of an IBM compatible computer having a 386 processor.
It is an object of the invention to provide a speech detection device which can be incorporated in a telephone, a fax apparatus or a telephone answering, device, in which a substantially lower computing; power is available than in a 386 processor. This has the advantage that an input signal can be monitored in the time domain and no fast Fourier transform is required by testing with the aid of the threshold value test means whether two frequency band signals contained in two frequency ranges of the input signal exceed an amplitude threshold value during a threshold time interval, it is determined whether the energy content of the input signal in these two frequency ranges exceeds an energy threshold value during a test time interval which contains the threshold time intervals. If this is the case, the evaluation means detect a speech signal in the input signal because signaling tones do not have any significant energy content in the two frequency ranges.
The invention provides the advantage that a reliable detection is also possible in the case of different types of speech signals contained in an input signal. A reliable detection is then also possible in the case of a speech signal from a speaker having a very low or a very high voice, which speech signal only has a significant energy content in the first and in the second frequency range, respectively.
The invention provides the advantage that both information from subfrequency bands and information from the first and the second frequency band is available to the evaluation means in order to determine whether a speech signal is detected, which enables allowance to be made for the overall energy distribution of the input signal.
The invention provides the advantage that information from at least two test time intervals is available to the evaluation means in order to determine whether the input signal contains a speech signal.
The invention provides the advantage that the filter means of a speech detection device are formed by only one digital filter whose filter coefficients are changed two times during a test time interval in order to obtain information from two frequency ranges of the input signal. This results in a speech detection device having low cost filter means.
The invention provides a communication device including telephone means and fax means, having a particularly reliable fax switch.
The invention provides a telephone answering device which terminates the storage of a speech signal contained in the input signal in a particularly reliable manner.
The invention will now be described in more detail with reference to three examples of use of an embodiment shown in the Figures and given by way of example, to which the invention is not limited.